SERI/TP-217 -3515 UC Category: 261 DE89009424
The Probable Source of Certain Spurious Frequencies Found in the Output of a Variable Speed Generating System Using Slip Recovery
P. Carlin
June 1989
Prepared for the 24th lntersociety Energy Conversion Engineering Conference Washington, D.C. 6-11 August 1989
Prepared under Task No • .WE933101
Solar Energy Research Institute A Division of Midwest Research Institute
1617 Cole Boulevard Golden, Colorado 80401-3393
Prepared for the U.S. Department of Energy Contract No. DE-AC02-83CH10093 NOTICE
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THE PROBABLE SOURCE OF CERTAIN SPURIOUS FREQUENCIES FOUND IN THE OUTPUT OF A VARIABLE SPEED GENERATING SYSTEM USING SLIP RECOVERY
Palmer W. Carlin Solar Energy Research Institute Golden, CO
ABSTRACT
Wind 0 30\300KVA As part of U.S. Department of Energy-sponsored re - Step-up search on wind energy, a Mod-O wind turbine was used to 7200V transformer drive a variable-speed, wound-rotor, induction generator. I Utility Energy resulting from the slip frequency voltage in the I generator rotor was rectified to DC, inverted back to H( utility frequency AC, and injected into the power line. : �I Rotor Spurious changing frequencies displayed in the generator i 1 ' output by a spectrum analyzer are caused by ripple on the I 9 0--
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enth were negative and positive, respectively, as would be expected. The probable source of this dissimilarity was the presence of some single-phase loads.
DISCUSSION
The most startling feature in Figure 3 is the pres ence of moving frequency components. In an ideal AC DC-AC link with perfectly ·constant DC, there is no way for frequency information to be transferred from one AC system to the other. The two AC systems on either side of the DC link should have completely unrelated frequen cies and reactive power flows. 0 300 420
Frequency in Hertz Because this plainly is not the case, let us calculate the ripple to be expected on the DC bus in the region about two-thirds of the way up the diagram. Because the Figure 2. Time history of the spectrum of the primary three-phase bridge rectifier connecting the variable gen current. erator rotor AC to the DC bus was a conventional six pulse type, we will see six pulses for every period of inpt!t Harmonics of utility frequency voltage. In addition, the generator (a four-pole type) pro Fund 3rd 5th 7th duced two electrical cycles for every mechanical slip cycle. From a recorder trace not shown here, we know the generator speed at this time was 2270 rpm. The rip I ple frequency on the DC link for this four-pole generator is therefore:
£5 (Mechanical Slip) x Ui (ij (2 electrical degrees) (6 switches)
Suppressing the skew allows the symmetries of the spec We immediately look for the complementary lower L trum to be more easily deduced. sidebands and find a weak one, 11, near the bottom of the spectrogram where the generator speed was not far It is easy to identify the third harmonic of the 60 Hz above synchronous and, therefore, the rotor voltage was waveform, as well as the fifth and seventh harmonics at fairly low. However, the ripple or modulating frequency 300Hz and 420Hz, respectively. They are the vertical soon exceeds the (60 Hz) carrier at "A," which implies a lines of Figure 3, denoting that the frequency is invariant "negative" frequency for the lower sideband. If we take with respect to time, the vertical axis. the frequency component that appears to continue from where this lower sideband went negative and "unfold" it Since the DC-to-AC conversion was done with a 12- around zero into the negative region, we observe the pulse inverter circuit, harmonics lower than the eleventh symmetry with the upper sideband. In the entire region, should theoretically be missing. Therefore, the phase the frequency difference between either sideband and the currents were very carefully examined, and it was found fundamental carrier is now always the calculated ripple that they were sufficiently dissimilar to account for the frequency. harmonics observed. The third harmonic consisted mostly of positive sequence component, and the ninth consisted The ripple waveform on the current of the DC link mostly of negative sequence component. Fifth and sev- is formed from six sinusoidal positive peaks of the three-
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phase bridge and, therefore, has not only odd but also even frequency components. It is not surprising, then, to find upper and lower sidebands, U 12 and L 12, with fre quency differences of twice the ripple frequency (i.e., a second harmonic). In Figure 4 we see again the lower sideband, which must be unfolded to display its symmetry with respect to its upper sideband. Also, when the fre quency changes, we note as a corroborating fact that these second harmonic sidebands change at twice the rate of the first harmonic sidebands described above. Note their comparative slopes in Figure 3.
Because the modulating frequency contains all har monics of the ripple frequency, it is not surprising to find traces of a third harmonic, U and L • Notice in Fig l3 13 ure 3 that these third harmomcs change three times as fast as the fundamental.
The reader has probably noticed that even if all the previously mentioned components are removed from the given spectrum, a number of frequencies remain. If we now assume that the constant frequency third, fifth, and seventh harmonics can act as carriers, we can lift off Frequency in Hz. successive families of sidebands centered on these fre quencies until the entire spectrogram is explained. For example, the two pairs of sidebands on the third harmonic Figure 5. Upper and lower sidebands on the utility of the utility frequency, u L and u L3 , which re third harmonic carrier caused by ripple fre 31 31 32 2 sult from the second and tlurdnarmonics of the DC link quency and its second harmonic. ripple frequency, are shown· in Figure 5.
It is easy, albeit tedious, to find segments of the ponent of the input to the inverter as well as measurable same pairs of sidebands enclosing the utility fifth and levels of the fundamental, second, and third harmonics of seventh harmonics. the "wild," or variable AC. The resulting mixing of these two sets of frequencies in the nonlinear characteristic of In summary, the output of a current link inverter by the inverter gives birth to all the preceding sidebands. its nature provides the desired fundamental 60Hz com Although we have no,t verified it quantitatively, the am ponent together with moderate levels of odd harmonics at plitudes of these sidebands show the expected variation. least up through the thirteenth. At the same time, the Because the amplitude of the AC voltage from the gener input three-phase bridge provides the desired DC com- ator rotor is proportional to slip frequency, and because the sideband amplitudes are proportional to the product
Harmonics of utility frequency of the amplitudes of the two frequencies that combine to Fund 3rd 5th 7th create them, it appears that the largest wandering fre \ quency grows and wanes in proportion to the magnitude I of slip frequency. \ � \ I \ Finally, one reason for concern about power system \Folded harmonics is that when power factor correction capaci \L12 tors are present, there is the possibility of resonance and I I I i2: the consequent overvoltage. This is especially true if I I I .� continuously varying components are present such as \ "0 those shown here. During these experiments, the Mod-O )_ \ c ' 8 did, in fact, have 100 kVAR of capacitance connected L13 ., I Folded I "' 1 I ., near the output of the inverter as shown in Figure . c "-..,_ / ��/\\ .e / ., Let us assume that a resonance could be recognized // E I ;: in our diagram by a noticeable increase in amplitude I when any variable frequency approached that resonant I frequency. The only possible candidate in this spectrum / / is the prominent first upper sideband of 60 Hz, U 11, which grows as its frequency increases from about 70 ffz to just under 180 Hz. This nearly 3 to 1 frequency ratio 0 60 180 300 420 is too wide to be a resonance. In addition none of the Frequency in Hz. other harmonics just below 180Hz shows any marked change in amplitude with frequency. From this we con Figure 4. Upper and lower sidebands of the second and cluded that resonances were not present in this particular third harmonics of the ripple frequency. experiment.
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RECOMMENDATIONS The other factor in the amplitude product is.what we have been calling the carrier·. Better balancing of the Although the preceding results seem to provide evi phases of the generating system would ameliorate all dence opposing the selection of the slip recovery princi harmonics below the eleventh; however, real life loads ple in a variable speed generating system, one should not are nearly always unbalanced •. immediately leap to that conclusion. Recall that the amplitudes of all these wandering components are propor The slip recovery system described here plainly has tional to the products of the amplitude of the utility fre the disadvantage that any resonance in the sweep range quencies and the residual ripple on the DC link. Two of the harmonics generated will certainly be excited from obvious improvements might be introduced. time to time. Other variable speed schemes involving alternative components such ·as pulse width and high The complete elimination of the ripple on the link is frequency resonant mode inverters are being tested. the ideal solution because all the wandering amplitudes Nevertheless, the slip recovery system remains the sim would go to zero. Realistically, a combination of addi plest and cheapest for "off-the-shelf" design, and it needs tional filtering of the DC link and a 12-pulse rectifier for to handle only a fraction of the total generated power. the DC could be introduced. Not only are the ripple har monics double frequency, but their relative amplitudes are less.